Optical transmitter having cascaded modulators

ABSTRACT

An optical data transmitter comprising two or more serially connected optical modulators, each driven using a respective DAC. The digital signals applied to the individual DACs are produced using different respective subsets of the set of bitstreams representing the digital waveform or data stream to be transmitted, with the bitstream subsets being selected, e.g., such that (i) each of the individual DACs is able to support the digital resolution and sampling rate needed for properly handling the subset of bitstreams applied thereto and (ii) differences between average driving powers applied to different optical modulators are relatively small. In different embodiments, the two or more serially connected optical modulators can be arranged for generating optical communication signals of different modulation formats, e.g., PSK, ASK, PAM, IM, and QAM. Some embodiments can advantageously be used for generating optical communication signals employing constellations of relatively large sizes, e.g., larger than 1000 symbols.

BACKGROUND Field

Various example embodiments relate to optical communication equipmentand, more specifically but not exclusively, to optical transmitters.

Description of the Related Art

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, the statements of thissection are to be read in this light and are not to be understood asadmissions about what is in the prior art or what is not in the priorart.

A fiber-optic communication system typically employs an optical datatransmitter at one end of an optical fiber line and an optical datareceiver at the other end of the optical fiber line. The telecomindustry and its suppliers develop, manufacture, sell, and use a largevariety of optical data transmitters and receivers for many differentcommunications applications.

SUMMARY OF SOME SPECIFIC EMBODIMENTS

Disclosed herein are various embodiments of an optical data transmittercomprising two or more serially connected optical modulators, eachdriven using a respective digital-to-analog converter (DAC). The digitalsignals applied to the individual DACs are produced using differentrespective subsets of the set of bitstreams representing the digitalwaveform or data stream to be transmitted, with the bitstream subsetsbeing selected such that, e.g., (i) each of the individual DACs is ableto support the digital resolution and sampling rate needed for properlyhandling the subset of bitstreams applied thereto and (ii) differencesbetween average driving powers applied to different ones of the seriallyconnected optical modulators can be relatively small. In someembodiments, the latter characteristic can advantageously be used, e.g.,to relax at least some of the transmitter circuits constraints and/or toimprove the transmitter operability and calibration procedures.

In different embodiments, the two or more serially connected opticalmodulators can be arranged for generating optical data signals ofdifferent modulation formats, e.g., Phase Shift Keying (PSK), AmplitudeShift Keying (ASK), Pulse Amplitude Modulation (PAM), IntensityModulation (IM), and Quadrature Amplitude Modulation (QAM). Someembodiments can advantageously be used for generating optical datasignals employing constellations of relatively large sizes, e.g., largerthan 250 symbols or even larger than 1000 symbols.

According to an example embodiment, provided is an apparatus comprisingan optical data transmitter that comprises first and second opticalmodulators and an electrical drive circuit, the first and second opticalmodulators being serially connected to generate a modulated opticalsignal in response to a first electrical drive signal applied to thefirst optical modulator and to a second electrical drive signal appliedto the second optical modulator; wherein the electrical drive circuit isconfigured to generate the first electrical drive signal using a firstsubset of an ordered set of parallel bitstreams and to generate thesecond electrical drive signal using a second non-overlapping subset ofthe ordered set, an order of the set being according to significance ofthe bitstreams to distances between data symbols carried by themodulated optical signal; and wherein one of the first and secondsubsets includes two of the bitstreams separated in said order by atleast one bitstream from another one of the first and second subsets.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and benefits of various disclosed embodimentswill become more fully apparent, by way of example, from the followingdetailed description and the accompanying drawings, in which:

FIG. 1 shows a block diagram of an optical data transmitter according toan embodiment;

FIGS. 2A-2C show different examples of a one-dimensional constellationbased on which a symbol mapper used in the optical data transmitter ofFIG. 1 can be programmed in some embodiments;

FIG. 3 graphically illustrates some features of optical modulators usedin the optical data transmitter of FIG. 1 according to an embodiment;and

FIG. 4 shows a block diagram of an optical data transmitter according toanother embodiment.

DETAILED DESCRIPTION

Higher-order modulation can advantageously be used to increase theeffective data-transmission rate. For example, theQuadrature-Phase-Shift-Keying (QPSK) format (which can carry two bitsper symbol) was used at the onset of coherent optical transmission.Later, optical 16-ary Quadrature Amplitude Modulation (16-QAM, which cancarry four bits per symbol) became an often-used modulation format.Presently, the 64-QAM modulation format (which can carry six bits persymbol) is being used in some optical-communications applications. It isbelieved that this general trend toward larger constellation sizes willcontinue, at least in the near future, to meet the market demands withrespect to optical data-transmission volume and throughput.

Optical modulation employing a relatively large constellation can beimplemented using a digital-to-analog converter (DAC) having anappropriately high resolution that enables reliable generation of andswitching between multiple discrete signal levels at a desired rate.However, some large constellations may currently be beyond the technicalcapability of even a state-of-the-art single DAC at high symbol rates,e.g., at least for DACs implemented using the complementarymetal-oxide-semiconductor (CMOS) technology.

The above and possibly some other related problems in the state of theart can be addressed, e.g., using at least some embodiments disclosedherein. In an example embodiment, an optical data transmitter comprisestwo or more serially connected optical modulators, each driven using acorresponding dedicated DAC. The digital signals applied to theindividual DACs are produced using different respective subsets of theset of bitstreams representing the digital waveform to be transmitted,with the bitstream subsets being selected, e.g., such that (i) each ofthe individual DACs has sufficient digital resolution and sampling rateneeded for properly handling the subset of bitstreams applied theretoand (ii) differences between average driving powers applied to differentones of the serially connected optical modulators can be made relativelysmall. The latter characteristic can advantageously be used, e.g., torelax at least some of the chip-manufacturing constraints and/or toimprove the transmitter operability and calibration procedures.

FIG. 1 shows a block diagram of an optical data transmitter 100according to an embodiment. Transmitter 100 comprises a laser 110 and anoptical modulator 120. In operation, laser 110 generates an opticalcarrier 112 in a conventional manner. Optical modulator 120 thenmodulates optical carrier 112 in response to an input data signal 102,thereby generating a modulated optical output signal 128 having encodedthereon the data supplied by the input data signal. In an exampleembodiment, the input data signal 102 can be an electrical binary signalcomprising a sequence of binary “zeros” and “ones.” The modulatedoptical output signal 128 can be applied, e.g., to an optical fiber linefor transmission therethrough to a remote optical data receiver (notexplicitly shown in FIG. 1).

Optical modulator 120 comprises serially connected optical modulators122 and 126 and an electrical drive circuit 130. Optical modulator 122operates to modulate optical carrier 112 in response to an electricaldrive signal 114 applied thereto by drive circuit 130, therebygenerating a corresponding modulated optical signal 124. Opticalmodulator 126 operates to further modulate optical signal 124 inresponse to an electrical drive signal 116 applied thereto by drivecircuit 130, thereby generating the modulated optical output signal 128.Electrical drive signals 114 and 116 can be generated by drive circuit130 in response to the input data signal 102, e.g., as described below.Depending on the embodiment, optical modulators 122 and 126 can beamplitude modulators, phase modulators, intensity modulators, etc.

In an example embodiment, electrical drive circuit 130 comprises aserial-to-parallel (S/P) converter 132, a symbol mapper 136, a digitalwaveform generator 140, a bitstream router 146, and DACs 154 and 156interconnected as indicated in FIG. 1.

S/P converter 132 is configured to convert the input data signal 102into M parallel bitstreams, which are labeled in FIG. 1 using thereference numerals 134 ₁-134 _(M), where M is an integer greater thanone. In an example embodiment, the conversion process can be implementedin S/P converter 132 in accordance with the following example processingsteps: (step 1) receiving M bit values by way of the input data signal102; (step 2) directing the first of the M received bit values tobitstream 134 ₁; (step 3) directing the second of the M received bitvalues to bitstream 134 ₂; . . . ; and (step M+1) directing the M-th ofthe M received bit values to bitstream 134 _(M). The processing steps(1) through (M+1) may be repeated, e.g., for each period of M clockcycles of the input data signal 102. Due to the deserializationperformed in this manner, the bitstreams 134 ₁-134 _(M) may be clockedat a lower rate than the input data signal 102. In each clock cyclethereof, the bitstreams 134 ₁-134 _(M) provide a corresponding bit-word,A(k), to symbol mapper 136, where k is the time-slot index. Eachbit-word A(k) has M bits carried by the bitstreams 134 ₁-134 _(M),respectively, in the k-th clock cycle.

In an example embodiment, symbol mapper 136 can be programmed based on aselected one-dimensional constellation. In operation, symbol mapper 136converts each bit-word A(k) into a corresponding bit-word S(k), whichrepresents the corresponding constellation symbol of the selectedone-dimensional constellation. Each of the bit-words S(k) generated inthis manner has N bits, where N is an integer, and N≥M. Using thisconstellation-based mapping, any bit-word A(k) provided by thebitstreams 134 ₁-134 _(M) in a k-th time slot can be converted into acorresponding bit-word S(k), with the N bit values thereof being appliedto bitstreams 138 ₁-138 _(N), respectively, in the k-th time slotthereof. In some embodiments, symbol mapper 136 can be implemented usinga look-up table (LUT) having 2^(M) entries, wherein each M-bit value islinked with a corresponding unique N-bit value.

FIGS. 2A-2C show different examples of the one-dimensional constellationbased on which symbol mapper 136 can be programmed in some embodiments.A person of ordinary skill in the art will understand that otherone-dimensional constellations can similarly be used to program symbolmapper 136 in other embodiments.

FIG. 2A graphically illustrates an 8-ary Pulse-Amplitude-Modulation(8-PAM) constellation 210 that can be used to program symbol mapper 136according to an embodiment. The eight constellation points ofconstellation 210 are all located on the I-axis of the complex I-Qplane. Each of the constellation points is used to encode three bits,i.e., M=3. An example of 3-bit bit-words assigned to differentconstellation points of constellation 210 is shown above the I-axis inFIG. 2A. The relative amplitudes corresponding to the differentconstellation points of constellation 210 are shown below the I-axis inFIG. 2A and can be represented by the real numbers −7, −5, −3, −1, +1,+3, +5, and +7. A person of ordinary skill in the art will understandthat constellation 210 can be used, e.g., in an embodiment oftransmitter 100 in which optical modulators 122 and 126 are bothamplitude modulators.

In the illustrated embodiment, the constellation-point labeling is inaccordance with a reflected double-Gray mapping scheme, in which theconstellation points located in the positive I-half of constellation 210have binary-amplitude labels (i.e., the labels that do not include thesign bit) generated using conventional double-Gray mapping, while theconstellation points located in the negative I-half of the constellationhave binary labels generated by flipping the sign bits of thecorresponding constellation points located in the positive I-half. Withthis type of mapping, the amplitude labels of the constellation pointsare symmetric, and the sign bits of the constellation points areanti-symmetric with respect to the origin of the I-axis. In somealternative embodiments, a similar approach can be used to generatebinary labels for a constellation or a constellation portion that usesthe Q dimension of the complex I-Q plane.

FIG. 2B graphically illustrates a unipolar 4-PAM constellation 220 thatcan be used to program symbol mapper 136 according to anotherembodiment. The four constellation points of constellation 210 are alllocated in the positive half of the I-axis. Each of the constellationpoints is used to encode two bits, i.e., M=2. The 2-bit bit-wordsassigned to different constellation points of constellation 220 areshown above the I-axis in FIG. 2B. The relative amplitudes correspondingto the different constellation points of constellation 220 are shownbelow the I-axis in FIG. 2B and can be represented by the real numbers+1, +3, +5, and +7. A person of ordinary skill in the art willunderstand that constellation 220 can be used, e.g., in an embodiment oftransmitter 100 in which optical modulators 122 and 126 are bothintensity modulators.

FIG. 2C graphically illustrates an 8-ary Phase-Shift-Keying (8-PSK)constellation 230 that can be used to program symbol mapper 136according to yet another embodiment. The eight constellation points ofconstellation 230 are all located on a circle having a radius R=1 andcentered on the origin of the complex I-Q plane. Each of theconstellation points is used to encode three bits, i.e., M=3. The 3-bitbit-words assigned to different constellation points of constellation230 are shown next to the constellation points in FIG. 2C. Thisparticular constellation-point labeling is in accordance with Graymapping. The relative phases corresponding to the differentconstellation points of constellation 210 are listed in the legend shownin FIG. 2C and can be represented by the real numbers 0, 45, 90, 135,180, 225, 270, and 315. A person of ordinary skill in the art willunderstand that constellation 230 can be used, e.g., in an embodiment oftransmitter 100 in which optical modulators 122 and 126 are both phasemodulators.

Referring back to FIG. 1, digital waveform generator 140 is configuredto convert the sequence of bit-words S(k) supplied by way of bitstreams138 ₁-138 _(N) into a corresponding digital waveform W(j) suitable fordriving the employed type of optical modulators 122 and 126, where j isthe time-slot index. Each of the bit-words of the digital waveform W(j)generated in this manner has P bits, where P is an integer, and P≥N.Using the processing performed in the digital waveform generator 140,each bit-word S(k) provided by the bitstreams 138 ₁-138 _(N) in a k-thtime slot can be converted into one or more corresponding bit-wordsW(j), with the P bit values of each bit-word W(j) being applied tobitstreams 142 ₁-142 _(P), respectively, in the j-th time slot thereof.The indices p (where p=1, 2, . . . , P) indicate the ordering of bits ofthe bit-words W(j) in the order of relative bit significance. Forexample, the bitstream 142 ₁ can carry the most significant bits (MSBs)of the bit-words W(j). In some embodiments, the MSB can be a sign bit.The bitstream 142 ₂ can carry the second most significant bits of thebit-words W(j), and so on. The bitstream 142 _(P) can carry the leastsignificant bits (LSBs) of the bit-words W(j). In an example embodiment,the number P can be greater than two. The upper limit on the number Ptypically depends on the digital resolution(s) supported by the DACs 154and 156.

Herein, a first bitstream is more significant than a second bitstream,e.g., because, in response to a variation of a value of the firstbitstream, transmitter 100 is able to produce a larger distance betweensome data symbols modulated onto an optical carrier than in response toa variation of a value of the second bitstream. As an example, FIG. 2Ashows a mapping of triplets of binary digits onto data symbols carriedby an optical carrier. For such a mapping, a variation of the bitstreamfor the left-most binary digit of the triplets can produce a maximumdistance of 14 between such data symbols. For comparison, a variation ofthe bitstream for the center binary digit of the triplets can produce amaximum distance of 4 between such data symbols. For further comparison,a variation of the bitstream for the right-most binary digit of thetriplets can produce a maximum distance of 2 between such data symbols.Thus, for the shown mapping, the bitstream of the let-most digit of thetriplets is the most significant, the bitstream of the right-most digitof the triplets is the least significant, and the bitstream of thecenter digit of the triplets has a relative significance between thoseof the other two bitstreams.

In an example embodiment, the processing implemented in the digitalwaveform generator 140 may include one or more digital-processingoperations from the following nonexclusive list: (i) scaling; (ii)interpolation; (iii) oversampling; (iv) pulse shaping; (v)pre-distortion; and (vi) pre-compensation. These digital-processingoperations can be implemented, e.g., in a conventional manner, as knownto persons of ordinary skill in the pertinent art. For example, ascaling operation may include multiplication of a digital value by aselected multiplication factor. An interpolation operation may includeobtaining one or more additional digital values (bit-words) based on thedigital values provided by two or more bit-words S(k) corresponding todifferent time-slot indices k, the additional digital values beingestimates of the function S(t) corresponding to the function S(k), wheret is allowed to have intermediate values in a range between saiddifferent time indices k. Unlike the time-slot indices k, the time tused for the function S(t) is not necessarily integer valued, i.e., canbe real-valued. A pulse-shaping operation may include constructingand/or changing a pulse-shaped waveform, e.g., to make the resultingoptical signal 128 better suited for transmission through thecorresponding optical communication channel. An oversampling operationmay include increasing the number of output digital samples compared tothe number of input digital samples. Oversampling is typically usedtogether with interpolation and/or pulse shaping and results in theoutput bitstreams 142 ₁-142 _(P) that are clocked at a higher rate thanthe corresponding input bitstreams 138 ₁-138 _(N). This feature isalready indicated above by the use of differently labeled respectivetime-slot indices, i.e., k versus j, for the streams of bit-words S(k)and W(j), respectively. A pre-distortion operation may involve changinga digital waveform in a manner that counteracts signal distortionsimposed by the optoelectronic front end of transmitter 100. For example,such signal distortions may be caused by a nonlinearity of the transferfunction(s) of one or both of optical modulators 122 and 126. Apre-compensation operation may involve changing a digital waveform in amanner that counteracts signal distortions imposed by the correspondingoptical communication channel. An example pre-compensation operation maybe directed at substantially cancelling, at the remote optical datareceiver, the effects of chromatic dispersion in the optical fiber lineconnecting the remote optical data receiver to transmitter 100.

Bitstream router 146 operates to: (i) direct a first subset ofbitstreams 142 ₁-142 p to DAC 154; and (ii) direct a second subset ofbitstreams 142 ₁-142 p to DAC 156. In an example embodiment, the firstand second subsets are non-overlapping, i.e., have no bitstreams 142 incommon. In one embodiment, the first and second subsets combinedtogether have all of the bitstreams 142 ₁-142 p, i.e., no bitstream 142is dropped by bitstream router 146. In another embodiment, at least onebitstream 142 may be dropped by bitstream router 146. The bitstreams ofthe first subset are labeled in FIG. 1 using the reference numerals 148₁-148 _(U). The bitstreams of the second subset are labeled in FIG. 1using the reference numerals 150 ₁-150 _(V). Herein, U and V arepositive integers, and U+V ≤P.

The indices u (where u=1, 2, . . . , U) order the bitstreams 148 ₁-148_(U) to preserve the relative significance of the correspondingbitstreams that exists in the bitstream set 142 ₁-142 _(P). For example,bitstream 148 ₁ has relatively more-significant bits than bitstream 148₂. Bitstream 148 ₂ has relatively more-significant bits than bitstream148 ₃, and so on. Similarly, the indices v (where v=1, 2, . . . , V)order the bitstreams 150 ₁-150 _(V) to preserve the relativesignificance of the corresponding bitstreams that exists in thebitstream set 142 ₁-142 _(P). For example, bitstream 150 ₁ hasrelatively more-significant bits than bitstream 150 ₂. Bitstream 150 ₂has relatively more-significant bits than bitstream 150 ₃, and so on.

In one embodiment, bitstream router 146 can be a fixed router, i.e., thepartition of the bitstreams 142 ₁-142 _(P) into the first and secondsubsets is fixed and static.

In another embodiment, bitstream router 146 can be a reconfigurablerouter (e.g., a P×(U+V) switch). In this case, a control signal 144generated by an appropriate electronic controller can be used to changethe partition of the bitstreams 142 ₁-142 p into the first and secondsubsets. For example, in various embodiments, bitstream router 146 maybe designed to enable one or more partition changes from the followingnonexclusive list: (i) changing the numbers U and/or V; (ii) moving oneor more selected bitstreams 142 from the first subset to the secondsubset; (iii) moving one or more selected bitstreams 142 from the secondsubset to the first subset; (iv) dropping one or more selectedbitstreams 142; and (v) adding a previously dropped bitstream 142 toeither the first subset or the second sub set.

As a non-limiting example, FIG. 1 shows one possible configuration ofbitstream router 146 corresponding to an even value of P, and thenumbers U=V=P/2. In this configuration, the odd-indexed bitstreams 142are directed to DAC 154, and the even-indexed bitstreams 142 aredirected to DAC 156. More specifically, the bitstreams 142 ₁, 142 ₃, . .. , and 142 _(P-1) are directed to DAC 154 as bitstreams 148 ₁, 148 ₂, .. . , and 148 _(U), respectively. Similarly, the bitstreams 142 ₂, 142₄, . . . , and 142 p are directed to DAC 156 as bitstreams 150 ₁, 150 ₂,. . . , and 150 _(V), respectively. Functionally, this routingconfiguration can be viewed as an operation that decomposes the digitalwaveform W(j) into two digital waveforms of lower (coarser) effectivedigital resolution. These two digital waveforms are combinable to fullyor substantially fully reconstruct the corresponding original digitalwaveform W(j), i.e., the decomposition may be reversible. In an exampleembodiment, the above-described interleaved allocation of the bitstreams142 to DACs 154 and 156 can beneficially result in a relatively small(e.g., approximately 6-dB) driving-power difference between the opticalmodulators 122 and 126, with this driving-power difference (expressed indB) being substantially independent of the number P.

In at least some embodiments, bitstream router 146 can be configured(statically or dynamically) to implement one or more of the followingrouting features:

-   -   (A) at least one bitstream 142 of the first subset has        relatively less-significant bits than at least one bitstream 142        of the second subset, and at least one bitstream 142 of the        first subset has relatively more-significant bits than at least        one bitstream 142 of the second subset;    -   (B) the first subset or the second subset includes both        bitstream 142 ₁ and bitstream 142 _(P), i.e., both the MSBs and        LSBs of the digital waveform We);    -   (C) the first subset has all odd-indexed bitstreams 142, and the        second subset has all even-indexed bitstreams 142, e.g.,        bitstream router 146 acts as a de-interleaving de-multiplexer;    -   (D) one of the first and second subsets includes two bitstreams        142 separated in the index order by at least one bitstream 142        from another one of the first and second subsets; and    -   (E) U≠V.

DAC 154 operates to convert the digital waveform applied thereto by thebitstreams 148 ₁-148 _(U) into a corresponding analog waveform, therebygenerating the electrical drive signal 114. In some embodiments,electrical drive signal 114 may be amplified and/or dc-biased to placeoptical modulator 122 into a proper operating point on its transferfunction, e.g., as known in the pertinent art.

DAC 156 similarly operates to convert the digital waveform appliedthereto by the bitstreams 150 ₁-150 _(V) into a corresponding analogwaveform, thereby generating electrical drive signal 116. In someembodiments, electrical drive signal 116 may also be amplified and/ordc-biased to place optical modulator 126 into a proper operating pointon its transfer function, e.g., as known in the pertinent art.

In some embodiments, symbol mapper 136 may be omitted (not present). Insuch embodiments, N=M.

In some embodiments, waveform generator 140 may be omitted (notpresent). In such embodiments, P=N.

In some embodiments, both symbol mapper 136 and waveform generator 140may be omitted (not present). In such embodiments, P=M.

In some embodiments, the number M is greater than six, i.e. M>6.

In some embodiments, optical modulator 120 can be modified in arelatively straightforward manner to have three or more opticalmodulators serially connected between laser 110 and the optical outputport of transmitter 100 to which the external optical fiber line isconnected. This modification may include (i) adding one or more DACs,e.g., one additional DAC per added, serially connected optical modulatorand (ii) connecting bitstream router 146 to route some of the bitstreams142 to the added DAC(s).

FIG. 3 graphically illustrates some features of optical modulator 120according to an embodiment. More specifically, the features illustratedin FIG. 3 correspond to an embodiment in which M=4 and opticalmodulators 122 and 126 of FIG. 1 are amplitude modulators. In thisexample, bitstream router 146 has a routing configuration analogous tothat explicitly shown in FIG. 1.

The third (rightmost) column in FIG. 3 graphically shows the discreteamplitude levels of optical output signal 128 (also see FIG. 1). Theseamplitude levels are in accordance with the shown 16-PAM constellation330. The corresponding sixteen amplitude values can be represented bythe real numbers −15, −13, −11, . . . , +13, +15, as indicated in FIG.3.

The first (leftmost) column in FIG. 3 graphically shows the discreteamplitude levels of optical signal 124 outputted by the first opticalmodulator 122 of FIG. 1 when the optical carrier 112 is a continuouswave (CW) signal (also see FIG. 1). These amplitude levels are inaccordance with the shown 4-PAM constellation 310. The correspondingfour amplitude values can be represented by the real numbers −10, −6,+6, and +10, as indicated in FIG. 3.

The second (middle) column in FIG. 3 graphically shows the discreteamplitude levels of the optical signal outputted by the second opticalmodulator 126 (FIG. 1) when the optical input thereto is a CW signal.These amplitude levels are in accordance with the shown 4-PAMconstellation 320. The corresponding four amplitude values can berepresented by the real numbers −5, −3, +3, and +5, as indicated in FIG.3. When optical modulator 126 so configured is connected to receiveoptical signal 124 illustrated in the first column of FIG. 3, e.g., fromthe first optical modulator 122 of FIG. 1, optical modulator 126 acts tosplit four ways each of the amplitude levels of said optical signal 124,thereby generating the optical output signal 128 illustrated in thethird column of FIG. 3. For example, the amplitude level −10 is splitfour ways in this manner to produce the amplitude levels −15 (=−10−5),−13 (=−10−3), −7 (−−10+3), and −5 (=−10+5) for the optical output signal128. The amplitude level −6 is similarly split four ways to produce theamplitude levels −11 (=−6−5), −9 (=−6−3), −3 (=−6+3), and −1 (=−6+5) forthe optical output signal 128. Similar splitting is produced for thepositive amplitudes +6 and +10 of said optical signal 124.

Note that the 4-PAM constellation 310 shown in the first column of FIG.3 is non-uniformly spaced, i.e., different pairs of adjacentconstellation points may have different respective spacings. Forexample, the two constellation points corresponding to the amplitudelevels +6 and +10 have a relative spacing of four units, whereas the twoconstellation points corresponding to the amplitude levels +6 and −6have a relative spacing of twelve units. This is a factor-of-threedifference in the spacing. The 4-PAM constellation 320 shown in thesecond column of FIG. 3 is also non-uniformly spaced. For example, thetwo constellation points corresponding to the amplitude levels +3 and +5have a relative spacing of two units, whereas the two constellationpoints corresponding to the amplitude levels +3 and −3 have a relativespacing of six units. In contrast, the 16-PAM constellation 330 shown inthe third column of FIG. 3 is uniformly spaced, i.e., any pair ofadjacent constellation points therein has the same spacing. In the shownexample, this uniform (constant) spacing is two units.

A person of ordinary skill in the art will readily understand thatsimilar non-uniformly spaced constellations can be used in an embodimentin which optical modulators 122 and 126 are phase modulators or in anembodiment in which optical modulators 122 and 126 are intensitymodulators.

In one possible embodiment, the electrical drive signals 114 and 116that can result in the optical signals corresponding to the two 4-PAMconstellations shown in FIG. 3 may be generated as follows. Theelectrical drive circuit 130 shown in FIG. 1 can be modified to haveboth symbol mapper 136 and waveform generator 140 removed. As a result,bitstream router 146 is connected to receive the bitstreams 134 ₁-134 ₄(herein P=M=4, as already mentioned above). The configuration ofbitstream router 146 may be similar to that explicitly shown in FIG. 1.In this configuration, bitstream router 146 directs the bitstreams 134 ₁and 134 ₃ to DAC 154 and also directs the bitstreams 134 ₂ and 134 ₄ toDAC 156. DAC 154 is configured to generate the electrical drive signal114 in accordance with Eq. (1):V ₁ =V ₀×(8×(−1)^(a)+2×(−1)^(c))  (1)where V₁ is the voltage of signal 114; V₀ is a constant; a is the bitvalue provided by the bitstream 134 ₁; and c is the bit value providedby the bitstream 134 ₃. DAC 156 is configured to generate the electricaldrive signal 116 in accordance with Eq. (2):V ₂ =V ₀×(4×(−1)^(b)+(−1)^(d))  (2)where V₂ is the voltage of signal 116; b is the bit value provided bythe bitstream 134 ₂; and d is the bit value provided by the bitstream134 ₄. It can easily be verified that nominally identical opticalmodulators 122 and 126 having a linear transfer function can produce theoptical signals illustrated in the three columns of FIG. 3 when drivenusing the voltages V₁ and V₂ generated in accordance with Eqs. (1) and(2).

FIG. 4 shows a block diagram of an optical data transmitter 400according to another embodiment, e.g., as an in-phase andquadrature-phase optical modulator. Transmitter 400 is constructed usingmany of the elements used in transmitter 100 (FIG. 1). These elementsare labeled in FIG. 4 using the same reference numerals as in FIG. 1.

Transmitter 400 comprises two instances of optical modulator 120, whichare labeled 120 ₁ and 120 ₂, respectively. In this embodiment, opticalmodulators 120 ₁ and 120 ₂ can be amplitude modulators connected inparallel between an optical splitter 402 and an optical combiner 408 asindicated in FIG. 4. Optical splitter 402 and optical combiner 408 canbe implemented using nominally identical optical components, e.g., twoinstances of a symmetric 3-dB optical power splitter.

In operation, optical splitter 402 splits the optical output of laser110 into two portions, which are labeled 112 ₁ and 112 ₂, respectively.Optical modulator 120 ₁ modulates optical signal 112 ₁ in response to aninput data signal 102 ₁, e.g., as described in reference to FIG. 1,thereby generating a corresponding modulated optical output signal 128₁. Optical modulator 120 ₂ similarly modulates optical signal 112 ₂ inresponse to a different input data signal 102 ₂, thereby generating acorresponding modulated optical output signal 128 ₂. A phase shifter 404applies about a 90-degree relative phase shift, e.g., with a precisionof ±5 degrees or preferably ±1 degree, to signal 128 ₂. Optical combiner408 then combines the resulting phase-shifted signal with signal 128 ₁,thereby generating a corresponding modulated optical output signal 428.The modulated optical output signal 428 can be applied, e.g., to anoptical fiber line for transmission therethrough to a remote opticaldata receiver (not explicitly shown in FIG. 4).

In an example embodiment, optical modulator 120 ₁ can be configured togenerate the modulated optical output signal 128 ₁ in accordance with anM₁-PAM constellation. Optical modulator 120 ₂ can similarly beconfigured to generate the modulated optical output signal 128 ₂ inaccordance with an M₂-PAM constellation. A person of ordinary skill inthe art will understand that, in this case, the corresponding modulatedoptical output signal 428 is generated in accordance with an (M₁×M₂)-QAMconstellation. In some embodiments, the numbers M₁ and M₂ may bedifferent. In some other embodiments, M₁=M₂.

According to an example embodiment disclosed above, e.g., in the summarysection and/or in reference to any one or any combination of some or allof FIGS. 1-4, provided is an apparatus comprising an optical datatransmitter (e.g., 100, FIG. 1) that comprises first and second opticalmodulators (e.g., 122, 126, FIG. 1) and an electrical drive circuit(e.g., 130, FIG. 1), the first and second optical modulators beingserially connected to generate a modulated optical signal (e.g., 128,FIG. 1; 428, FIG. 4) in response to a first electrical drive signal(e.g., 114, FIG. 1) applied to the first optical modulator and to asecond electrical drive signal (e.g., 116, FIG. 1) applied to the secondoptical modulator; wherein the electrical drive circuit is configured togenerate the first electrical drive signal using a first subset (e.g.,148 ₁-148 _(U), FIG. 1) of an ordered set of parallel bitstreams (e.g.,142 ₁-142 p, FIG. 1) and to generate the second electrical drive signalusing a second non-overlapping subset (e.g., 150 ₁-150 _(U), FIG. 1) ofthe ordered set, an order of the set being according to significance ofthe bitstreams to distances between data symbols carried by themodulated optical signal; and wherein one of the first and secondsubsets includes two of the bitstreams separated in said order by atleast one bitstream from another one of the first and second subsets.

In some embodiments of the above apparatus, the electrical drive circuitis configured to generate the ordered set of parallel bitstreams inresponse to input data (e.g., 102, FIG. 1); and wherein the opticalmodulators are configured to produce the modulated optical signal tocarry a sequence of constellation symbols encoding said input data.

In some embodiments of any of the above apparatus, the electrical drivecircuit comprises a bitstream router (e.g., 146, FIG. 1) to route thefirst subset of the parallel bitstreams to a first digital-to-analogconverter (e.g., 154, FIG. 1) and to route the second subset of theparallel bitstreams to a second digital-to-analog converter (e.g., 156,FIG. 1), the first digital-to-analog converter being configured togenerate the first electrical drive signal in response to the firstsubset, the second digital-to-analog converter being configured togenerate the second electrical drive signal in response to the secondsubset.

In some embodiments of any of the above apparatus, the bitstream routercomprises a switch configured to change at least one of the first andsecond subsets in response to a control signal (e.g., 144, FIG. 1).

In some embodiments of any of the above apparatus, the first opticalmodulator is an optical amplitude modulator, an optical intensitymodulator, or an optical phase modulator.

In some embodiments of any of the above apparatus, the second opticalmodulator is an optical amplitude modulator, an optical intensitymodulator, or an optical phase modulator.

In some embodiments of any of the above apparatus, the electrical drivecircuit comprises a serial-to-parallel converter (e.g., 132, FIG. 1)connected to output the ordered set of parallel bitstreams in responseto input data (e.g., embodiment without 136 and 140, FIG. 1).

In some embodiments of any of the above apparatus, the electrical drivecircuit comprises: a serial-to-parallel converter (e.g., 132, FIG. 1)connected to output a plurality of second parallel bitstreams (e.g., 134₁-134 _(M), FIG. 1) in response to input data (e.g., 102, FIG. 1); and adigital circuit (e.g., 136/140, FIG. 1) configured to generate theordered set of parallel bitstreams in response to the plurality ofsecond parallel bitstreams.

In some embodiments of any of the above apparatus, the digital circuitcomprises at least one of a symbol mapper (e.g., 136, FIG. 1) and adigital waveform generator (e.g., 140, FIG. 1).

In some embodiments of any of the above apparatus, the parallelbitstreams of the ordered set are clocked at a higher rate than thesecond parallel bitstreams.

In some embodiments of any of the above apparatus, the ordered set ofparallel bitstreams has more bitstreams than the plurality of secondparallel bitstreams (e.g., P>M, FIG. 1).

In some embodiments of any of the above apparatus, the electrical drivecircuit is configured to operate at least one of the first and secondoptical modulators to produce at an optical output thereof symbols of aone-dimensional constellation (e.g., 310, 320, FIG. 3) whose symbols arenon-uniformly spaced.

In some embodiments of any of the above apparatus, the modulated opticalsignal is generated to carry symbols of a one-dimensional constellation(e.g., 330, FIG. 3) whose symbols are uniformly spaced.

In some embodiments of any of the above apparatus, the two of thebitstreams are a bitstream of most-significant bits and a bitstream ofleast-significant bits.

In some embodiments of any of the above apparatus, the first and secondsubsets have odd-indexed first bitstreams and even-indexed firstbitstreams, respectively, with bitstream indexing being consecutive andin said order.

In some embodiments of any of the above apparatus, the modulated opticalsignal is generated to carry symbols of a constellation whose symbolsdiffer from one another in at least one of amplitude and phase.

In some embodiments of any of the above apparatus, the electrical drivecircuit is configured to operate each of the first and second opticalmodulators to modulate an optical carrier according to a correspondingconstellation (e.g., 310, 320, FIG. 3) whose symbols are non-uniformlyspaced, the constellation corresponding to the first optical modulatorhaving a different minimum spacing of symbols than the constellationcorresponding to the second optical modulator.

In some embodiments of any of the above apparatus, bitstreams of thefirst subset are interleaved with bitstreams of the second subset in theorder of the ordered set.

In some embodiments of any of the above apparatus, the optical datatransmitter comprises a laser (e.g., 110, FIG. 1) connected to apply anoptical carrier (e.g., 112, FIG. 1) to an optical input of the firstoptical modulator, the first optical modulator being operated togenerate a corresponding modulated optical output signal (e.g., 124,FIG. 1) at an optical output thereof; and wherein the second opticalmodulator is connected to receive said corresponding modulated opticaloutput signal at an optical input thereof, the second optical modulatorbeing operated to further modulate said corresponding modulated opticaloutput signal to generate a corresponding further-modulated opticalsignal (e.g., 128, FIG. 1) at an optical output thereof.

In some embodiments of any of the above apparatus, the modulated opticalsignal (e.g., 428, FIG. 4) comprises said correspondingfurther-modulated optical signal.

According to another example embodiment disclosed above, e.g., in thesummary section and/or in reference to any one or any combination ofsome or all of FIGS. 1-4, provided is an apparatus comprising an opticaldata transmitter (e.g., 100, FIG. 1) that comprises first and secondoptical modulators (e.g., 122, 126, FIG. 1) and an electrical drivecircuit (e.g., 130, FIG. 1), the first and second optical modulatorsbeing serially connected to generate a modulated optical signal (e.g.,128, FIG. 1; 428, FIG. 4) in response to a first electrical drive signal(e.g., 114, FIG. 1) applied to the first optical modulator and to asecond electrical drive signal (e.g., 116, FIG. 1) applied to the secondoptical modulator; wherein the electrical drive circuit is configured togenerate the first electrical drive signal using a first subset (e.g.,148 ₁-148 _(U), FIG. 1) of an ordered set of parallel bitstreams (e.g.,142 ₁-142 p, FIG. 1) and to generate the second electrical drive signalusing a second subset (e.g., 150 ₁-150 _(U), FIG. 1) of the ordered set,the first and second subsets being non-overlapping subsets, the parallelbitstreams being ordered in an order of relative bit significance; andwherein one of the first and second subsets includes two of the firstbitstreams separated in said order by at least one bitstream fromanother one of the first and second subsets.

While this disclosure includes references to illustrative embodiments,this specification is not intended to be construed in a limiting sense.Various modifications of the described embodiments, as well as otherembodiments within the scope of the disclosure, which are apparent topersons skilled in the art to which the disclosure pertains are deemedto lie within the principle and scope of the disclosure, e.g., asexpressed in the following claims.

Unless explicitly stated otherwise, each numerical value and rangeshould be interpreted as being approximate as if the word “about” or“approximately” preceded the value or range.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain the nature of this disclosure may bemade by those skilled in the art without departing from the scope of thedisclosure, e.g., as expressed in the following claims.

The use of figure numbers and/or figure reference labels in the claimsis intended to identify one or more possible embodiments of the claimedsubject matter in order to facilitate the interpretation of the claims.Such use is not to be construed as necessarily limiting the scope ofthose claims to the embodiments shown in the corresponding figures.

Although the elements in the following method claims, if any, arerecited in a particular sequence with corresponding labeling, unless theclaim recitations otherwise imply a particular sequence for implementingsome or all of those elements, those elements are not necessarilyintended to be limited to being implemented in that particular sequence.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of thedisclosure. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments necessarilymutually exclusive of other embodiments. The same applies to the term“implementation.”

Unless otherwise specified herein, the use of the ordinal adjectives“first,” “second,” “third,” etc., to refer to an object of a pluralityof like objects merely indicates that different instances of such likeobjects are being referred to, and is not intended to imply that thelike objects so referred-to have to be in a corresponding order orsequence, either temporally, spatially, in ranking, or in any othermanner.

Unless otherwise specified herein, in addition to its plain meaning, theconjunction “if” may also or alternatively be construed to mean “when”or “upon” or “in response to determining” or “in response to detecting,”which construal may depend on the corresponding specific context. Forexample, the phrase “if it is determined” or “if [a stated condition] isdetected” may be construed to mean “upon determining” or “in response todetermining” or “upon detecting [the stated condition or event]” or “inresponse to detecting [the stated condition or event].”

Also for purposes of this description, the terms “couple,” “coupling,”“coupled,” “connect,” “connecting,” or “connected” refer to any mannerknown in the art or later developed in which energy is allowed to betransferred between two or more elements, and the interposition of oneor more additional elements is contemplated, although not required.Conversely, the terms “directly coupled,” “directly connected,” etc.,imply the absence of such additional elements. The same type ofdistinction applies to the use of terms “attached” and “directlyattached,” as applied to a description of a physical structure. Forexample, a relatively thin layer of adhesive or other suitable bindercan be used to implement such “direct attachment” of the twocorresponding components in such physical structure.

The description and drawings merely illustrate the principles of thedisclosure. It will thus be appreciated that those of ordinary skill inthe art will be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of thedisclosure and are included within its spirit and scope. Furthermore,all examples recited herein are principally intended expressly to beonly for pedagogical purposes to aid the reader in understanding theprinciples of the disclosure and the concepts contributed by theinventor(s) to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosure, as well as specific examples thereof, areintended to encompass equivalents thereof.

The functions of the various elements shown in the figures, includingany functional blocks labeled as “processors,” “controllers,” and/or“digital circuits” may be provided through the use of dedicated hardwareas well as hardware capable of executing software in association withappropriate software. When provided by a processor, the functions may beprovided by a single dedicated processor, by a single shared processor,or by a plurality of individual processors, some of which may be shared.Moreover, explicit use of the term “processor” or “controller” shouldnot be construed to refer exclusively to hardware capable of executingsoftware, and may implicitly include, without limitation, digital signalprocessor (DSP) hardware, network processor, application specificintegrated circuit (ASIC), field programmable gate array (FPGA), readonly memory (ROM) for storing software, random access memory (RAM), andnon volatile storage. Other hardware, conventional and/or custom, mayalso be included. Similarly, any switches shown in the figures areconceptual only. Their function may be carried out through the operationof program logic, through dedicated logic, through the interaction ofprogram control and dedicated logic, or even manually, the particulartechnique being selectable by the implementer as more specificallyunderstood from the context.

As used in this application, the term “circuitry” may refer to one ormore or all of the following: (a) hardware-only circuit implementations(such as implementations in only analog and/or digital circuitry); (b)combinations of hardware circuits and software, such as (as applicable):(i) a combination of analog and/or digital hardware circuit(s) withsoftware/firmware and (ii) any portions of hardware processor(s) withsoftware (including digital signal processor(s)), software, andmemory(ies) that work together to cause an apparatus, such as a mobilephone or server, to perform various functions); and (c) hardwarecircuit(s) and or processor(s), such as a microprocessor(s) or a portionof a microprocessor(s), that requires software (e.g., firmware) foroperation, but the software may not be present when it is not needed foroperation.” This definition of circuitry applies to all uses of thisterm in this application, including in any claims. As a further example,as used in this application, the term circuitry also covers animplementation of merely a hardware circuit or processor (or multipleprocessors) or portion of a hardware circuit or processor and its (ortheir) accompanying software and/or firmware. The term circuitry alsocovers, for example and if applicable to the particular claim element, abaseband integrated circuit or processor integrated circuit for a mobiledevice or a similar integrated circuit in server, a cellular networkdevice, or other computing or network device.

It should be appreciated by those of ordinary skill in the art that anyblock diagrams herein represent conceptual views of illustrativecircuitry embodying the principles of the disclosure.

What is claimed is:
 1. An apparatus comprising an optical datatransmitter that comprises first and second optical modulators and anelectrical drive circuit, the first and second optical modulators beingserially connected to generate a modulated optical signal in response toa first electrical drive signal applied to the first optical modulatorand to a second electrical drive signal applied to the second opticalmodulator; wherein the electrical drive circuit is configured togenerate the first electrical drive signal using a first subset of anordered set of parallel bitstreams and to generate the second electricaldrive signal using a second non-overlapping subset of the ordered set,an order of the set being according to significance of the bitstreams todistances between data symbols carried by the modulated optical signal;and wherein one of the first and second subsets includes two of thebitstreams separated in said order by at least one bitstream fromanother one of the first and second subsets.
 2. The apparatus of claim1, wherein the electrical drive circuit is configured to generate theordered set of parallel bitstreams in response to input data; andwherein the optical modulators are configured to produce the modulatedoptical signal to carry a sequence of constellation symbols encodingsaid input data.
 3. The apparatus of claim 1, wherein the electricaldrive circuit comprises a bitstream router to route the first subset ofthe parallel bitstreams to a first digital-to-analog converter and toroute the second subset of the parallel bitstreams to a seconddigital-to-analog converter, the first digital-to-analog converter beingconfigured to generate the first electrical drive signal in response tothe first subset, the second digital-to-analog converter beingconfigured to generate the second electrical drive signal in response tothe second subset.
 4. The apparatus of claim 3, wherein the bitstreamrouter comprises a switch configured to change at least one of the firstand second subsets in response to a control signal.
 5. The apparatus ofclaim 1, wherein the electrical drive circuit comprises aserial-to-parallel converter connected to output the ordered set ofparallel bitstreams in response to input data.
 6. The apparatus of claim1, wherein the electrical drive circuit comprises: a serial-to-parallelconverter connected to output a plurality of second parallel bitstreamsin response to input data; and a digital circuit configured to generatethe ordered set of parallel bitstreams in response to the plurality ofsecond parallel bitstreams.
 7. The apparatus of claim 6, wherein thedigital circuit comprises at least one of a symbol mapper and a digitalwaveform generator.
 8. The apparatus of claim 6, wherein the parallelbitstreams of the ordered set are clocked at a higher rate than thesecond parallel bitstreams.
 9. The apparatus of claim 6, wherein theordered set of parallel bitstreams has more bitstreams than theplurality of second parallel bitstreams.
 10. The apparatus of claim 1,wherein the electrical drive circuit is configured to operate at leastone of the first and second optical modulators to produce at an opticaloutput thereof symbols of a one-dimensional constellation whose symbolsare non-uniformly spaced.
 11. The apparatus of claim 10, wherein themodulated optical signal is generated to carry symbols of aone-dimensional constellation whose symbols are uniformly spaced. 12.The apparatus of claim 1, wherein the two of the bitstreams are abitstream of most-significant bits and a bitstream of least-significantbits.
 13. The apparatus of claim 1, wherein the first and second subsetshave odd-indexed first bitstreams and even-indexed first bitstreams,respectively, with bitstream indexing being consecutive and in saidorder.
 14. The apparatus of claim 1, wherein the electrical drivecircuit is configured to operate each of the first and second opticalmodulators to modulate an optical carrier according to a correspondingconstellation whose symbols are non-uniformly spaced, the constellationcorresponding to the first optical modulator having a different minimumspacing of symbols than the constellation corresponding to the secondoptical modulator.
 15. The apparatus of claim 1, wherein bitstreams ofthe first subset are interleaved with bitstreams of the second subset inthe order of the ordered set.
 16. The apparatus of claim 1, wherein theoptical data transmitter comprises a laser connected to apply an opticalcarrier to an optical input of the first optical modulator, the firstoptical modulator being operated to generate a corresponding modulatedoptical output signal at an optical output thereof; and wherein thesecond optical modulator is connected to receive said correspondingmodulated optical output signal at an optical input thereof, the secondoptical modulator being operated to further modulate said correspondingmodulated optical output signal to generate a correspondingfurther-modulated optical signal at an optical output thereof.
 17. Theapparatus of claim 16, wherein the modulated optical signal comprisessaid corresponding further-modulated optical signal.